首先fruit存在可数的情况。
fruits is also sometimes used as the plural in the botanical sense:
berries, achenes, and nuts are all fruits; the fruits of this plant split into two parts.
下列是fruit作为可数或不可数的情况：
In the botanical and figurative senses, fruit is usually treated as uncountable:
a bowl of fruit; eat plenty of fruit; the tree provides fruit.
fruits is also sometimes used as the plural in the botanical sense:
berries, achenes, and nuts are all fruits; the fruits of this plant split into two parts.
When fruit is treated as uncountable in the botanical sense, a piece of fruit is often used as a singulative.
In senses other than the botanical or figurative ones derived from the botanical sense, the plural is fruits.
The culinary sense often does not cover true fruits that are savoury or used chiefly in savoury foods, such as tomatoes and peas. These are normally described simply as vegetables.
而这个句子中的情况符合第一条表述，所以作为不可数名词，不加s。
资料来源：维基百科